Hitherto, as a kind of direct printing apparatus for directly forming an image on a sheet, an apparatus which forms the image by using printing particles such as toner has been known. The direct printing apparatus comprises a developing device for bearing toner to serve development, the developing device having double rollers comprising a first roller and a second roller, a backing electrode opposed to the second roller of the developing device, and a printing head disposed between the backing electrode and the second roller, the printing head having a plurality of apertures and control electrode surrounding the plurality of apertures. To the first roller of the developing device is applied a voltage V1, while to the second roller is applied a voltage V2. To the backing electrode is applied a voltage V.sub.BE, thereby an electric field is generated between the second roller and the backing electrode for propelling the toner toward the backing electrode side from the second roller through the apertures. To the control electrode of the printing head is selectively applied either a printing voltage Vb having a reverse polarity to the toner or a non-printing voltage Vw having a same polarity as the toner.
FIG. 4(A) shows an electric potential distribution between the first roller, the second roller and the control electrode of the conventional direct printing apparatus. When applying the voltage Vb (for example, +300 volts) having a reverse polarity to the toner to the control electrodes, the toner on the second roller to which a voltage of, for example, V2=0 is applied (i.e., connected to the earth) is propelled. On the other hand, when applying the voltage Vw (for example, -50 volts) having a same polarity as the toner to the control electrodes, the toner is never propelled. Thus, when the applying voltage Vb, Vw to the control electrodes of the printing head are changed on the basis of a desired image signal, a toner image corresponding to the image signal is formed in the sheet.
If the toner charged quantity is large, the adhesive strength to the second roller becomes large. Therefore, the toner is difficult to be propelled, thereby desired image density is not obtained. So, in the case that the toner charged quantity is large, the following provision has been made. As shown in FIG. 4(B), the applied voltage V1 to the first roller is enlarged in comparison to the case that toner charged quantity is small. Thereby, movement of the toner to the second roller from the first roller is accelerated so that adhered toner on the second roller becomes large in quantity, i.e., becomes multilayered. Thus, the adhesive strength of the surface layer toner to the second roller becomes small so that the toner is easy to be propelled, thereby desired image density is obtained.
However, the surface layer of the multilayered toner on the second roller, as shown in FIG. 4(B), is elevated to the same electric potential (-200 volts) as that of the first roller. Thus, even at the non-printing portion, a few of the toner on the second roller is propelled, causing fog in the background of the image.